Physical and Chemical Exergy

The first term on the right-hand side of this equation expresses the amount of work available due to differences in pressure and temperature with the environment. The second term, the chemical exergy, expresses the amount of work available due to the differences in composition with respect to the environment. The superscript in Ex, expresses that the chemical exergy is considered at ambient conditions. 

---

The conditions of a natural gas reservoir are 30 MPa and 100°C. The gas, assumed to be pure methane, is spontaneously expanded to a pressure of 7MPa (Figure 8.1). Assuming that this expansion is adiabatic, calculate the amount of work that is lost in the process, and express it as a fraction of the originally available amount of work per mole of gas in the reservoir. Carry out this calculation while making a distinction between the physical and chemical exergy of the gas. 

In line with what was discussed in Chapter 6 with regard to the quality of the Joule, one can interpret Orwell [6], "All Joules are equal, but some Joules are more equal than others." This means that 1J of heat at 1000 K is more useful than, say, 1J of heat at 298 K. This is a direct consequence of the work available in these amounts of heat, as stated in Chapters 6 and 7, where precise definitions of physical and chemical exergy are given. A direct consequence of the second law of thermodynamics is that the available work (exergy) can never be utilized completely in real processes. Since all real processes are irreversible, every process step will produce a finite amount of lost work, thus diminishing the amount of useful work. 

Sometimes it is appropriate to consider the costs of physical and chemical exergy associated with a material stream separately. By denoting the average costs per unit of physical and chemical exergy by and respectively, the cost rate associated with stream j becomes... 

The quality of a material stream can be expressed using their physical and chemical exergy ... 

Total exergy of hydrogen is sum of the physical and chemical exergies. 

In the absence of nuclear, magnetic, electrical, and sirr-face tension effects, the total exergy of a system Esys can be divided into four components physical exergy Ef, kinetic exergy E , potential exergy E, and chemical exergy E ... 

Exergy is composed of two parts, which are physical exergy and chemical exergy, as discussed in Chapter III.5.2.1. Physical equations are calculated by the equation (III.62) for liquids and equation (III.63a) for gases. Chemical exergies are obtained from the table A. 1. 

In this chapter, we make preparations for performing a thermodynamic analysis of a process. The principles of such an analysis are defined first. From the calculation of the minimum, also called the ideal amount of work to perform a certain task, the convenience, not the necessity, of defining the concept of exergy is made plausible. Exergy can have a physical and a chemical component. The quality of the Joule is another convenient concept for a clear analysis and for conclusions on process performance. 

Table 6.2 lists exergy values for methane. It is clear from this table that methane carries an impressive amount of exergy as chemical exergy. Further, the table shows (1) the influence of increased pressure and temperature on the physical exergy and (2) that this latter contribution of exergy is nearly two orders smaller than the chemical contribution. Chemical exergy is the exclusive subject of Chapter 7. 

Exergy is a convenient concept if one wishes to assign a quantitative quality mark to a stream or a product. This quality mark expresses the maximum available work or potential to perform work because of its possible differences in pressure, temperature, and composition with the prevailing environment. The physical exergy, Exphys, only accounts for the differences in pressure and temperature the standard chemical exergy, Ex/hf.rn, accounts for the difference in composition with the environment at the environment s pressure and temperature. Thus... 

In the last chapter, the concepts of exergy and physical exergy, in particular, were introduced. This chapter deals with three other important concepts, namely, exergy of mixing, chemical exergy, and cumulative exergy consumption, and their numerical evaluation. 

If the thermodynamic efficiency of a process step is calculated, the chemical exergies should be excluded from the calculation if the process step does not include chemical conversions. If it does, it may be appropriate to distinguish between the physical and the chemical efficiency, itphys and T chem, °f the process step. 

N2 and H2 and the production of H2 from natural gas. Assume physical steps to have efficiencies of 10% and chemical steps of 60%. What is the cumulative exergy consumption of urea Separations take place with the help of compressors with an efficiency of 75%, running on electricity from an advanced natural gas-fired power station with a thermodynamic efficiency of 55%. 

Total exergy Ex of a multicomponent material stream consists of physical, chemical, and mixing parts. Disregarding kinetic and potential exergy contributions, the rate of exergy of a stream is Ex // T0S, Ex = hEx, where h is the molar flow rate of a stream and Ex the molar exergy. Similarly, H and W, are the stream enthalpy and entropy rates, respectively, and are based on reference conditions. T0 is the environmental temperature usually assumed as 298.15 K. 

The purpose of a gasifier is to convert the chemical exergy of a solid fuel into the chemical exergy of a gaseous fuel. Due to the chemical reactions involved, the physical exergy of the material streams increase between the inlet and the outlet. 

Physical exergy results from the deviation of temperature and pressure from the environmental values. Chemical exergy results from the deviation of the composition of a component in a system from the composition of the same component in the environment. Chemical exergy of a stream of matter can be defined as the maximum work (useful energy) that can be obtained from it in taking it to chemical equilibrium (of composition) with the environment. In an open system, the specific flow exergy exf is expressed by... 

The physical exergy Eph is equal to the maximum amount of work obtainable when a compound or mixture is brought from its temperature T and pressure P to environmental conditions, characterized by environmental temperature T and pressure Pq. The standard chemical exergy of a pure chemical compound Ech is equal to the maximum amount of work obtainable when a compound is brought from the environmental state, characterized by the environmental temperature To (298.15 K) and environmental pressure Po (1 atm), to the dead state, characterized by the same environmental conditions of temperature and pressure, but also by the concentration of reference substances in a standard environment. 

---